1. Field of the Invention
The present invention relates to an image-taking apparatus such as a video camera.
2. Description of the Related Art
For auto-focus control of a video camera, a so-called TV-AF method is predominantly used to perform control in which an object image is photoelectrically converted by an image-pickup device into a video signal, the sharpness of the video is detected in the video signal and is used as an AF evaluation value, and the position of a focus lens at which the AF evaluation value is at the peak is searched for.
As the AF evaluation value in the TV-AF method, a high-frequency component extracted from a video signal by a band pass filter for a certain band is typically used. When a normal object image is taken, the AF evaluation value is higher as a focus lens approaches an in-focus point (in-focus position) as shown in FIG. 2, and the highest level of the AF evaluation value corresponds to the in-focus point.
Another AF method is a TTL (Through The Lens) phase difference detection method. In the TTL phase difference detection method, a luminous flux passing through an exit pupil of an image-taking lens is divided into two which are received by a set of line sensors. Then, a displacement amount in signals output in accordance with the received light amounts, that is, a relative positional displacement amount in the direction in which the luminous flux is divided, is detected to directly determine a defocus amount of the image-taking lens. Thus, once a focus detection sensor is used to perform storage operation, the amount and direction of driving of a focus lens can be provided to allow focus adjustment operation at high speed.
In addition, the phase difference detection method includes an external metering phase difference detection type which does not use light passing through an image-taking lens. In the external metering phase difference detection method, a luminous flux from an object is divided into two which are received by a set of line sensors. Then, a displacement amount in signals output in accordance with the received light amounts, that is, a relative positional displacement amount in the direction in which the luminous flux is divided, is detected to determine the distance to the object with triangulation.
Other AF methods using an external metering sensor include a method in which an ultrasonic sensor is used to measure the distance to an object from the propagation speed of ultrasonic waves, a method in which an infrared sensor is used to perform triangulation, and the like.
In addition, a hybrid AF method is used by combining these AF schemes. In the hybrid AF method, for example, a focus lens is driven to a position close to an in-focus point with the TTL phase difference detection method, and then the focus lens is driven more accurately to the in-focus position with the TV-AF method (for example, see Japanese Patent Application Laid-Open No. 5-64056 (No. 1993-64056).
The aforementioned hybrid AF method, however, has the following disadvantage. Specifically, in the hybrid AF method, the final AF control is performed in the TV-AF method since the TV-AF method generally enables a more accurate search for an in-focal position. Conversely, the AF methods other than the TV-AF method provide lower focusing accuracy than the TV-AF method.
For this reason, when the focus lens is driven in an AF method other than the TV-AF method, the focus lens may pass the in-focus position at which the focus lens would stop in the TV-AF method. Thus, a video being taken involves unnatural focus changes in which focusing is once achieved, then slight blurring is caused, and thereafter, focusing is being achieved again by auto-focus in the TV-AF method.